The present invention relates to a device for reading bar codes on semiconductor surfaces in which a laser beam is reflected off the semiconductor surface with the bar code and received by a receiving system including a photodiode.
U.S. Pat. No. 4,585,931 discloses a device for reading bar codes on semiconductor surfaces by means of an optical transmit/receive system having transmitting and receiving optical axes which enclose an angle of 180.degree.-2.THETA., with the angle .THETA. being formed between the semiconductor surface and the respective optical axis. The angle .THETA. is selectable over a range from approximately 30.degree. to 60.degree. . A light source, which is not identified in detail in the patent, beams a bundle of light onto the semiconductor surface at the angle .THETA.. This bundle of light is reflected at the angle .THETA. at the polished semiconductor surface or, more precisely, in the region of the gaps between the bar codes, and travels through a lens and an aperture system onto a photodetector. The photocurrent generated in the photodetector charges an amplifier whose output signal is fed to a decoder. The result of the decoding process is indicated on a display.
By rotation of the semiconductor disc, the bar code is moved past and underneath the transmit/receive system. The transmitted light is temporarily scattered at the micro-roughnesses of the bars.
The drawback is here that the aperture system including the lens at the receiving end has only a small aperture. Therefore, no or only a little light reaches the photodetector in the region of the micro-roughnesses. Moreover, the light source radiates incoherent light so that optical filtering of the light in order to raise contrast is not possible. Additionally, slopes in the semiconductor surface adversely affect the intensity of the received signal.